1. Introduction
In the standard theory of relativity, physics is local in the sense that a postulate of locality permeates through the special and general theories of relativity. First, Lorentz invariance is extended in a pointwise manner to actual, namely, accelerated, observers in Minkowski spacetime. This
hypothesis of locality is then employed crucially in Einstein’s local principle of equivalence to render observers pointwise inertial in a gravitational field [
1]. Field measurements are intrinsically nonlocal, however. To go beyond the locality postulate in Minkowski spacetime, the past history of the accelerated observer must be taken into account. The observer in general carries the memory of its past acceleration. The deep connection between inertia and gravitation suggests that gravity could be nonlocal as well, and, in nonlocal gravity, the gravitational memory of past events must then be taken into account. Along this line of thought, a classical nonlocal generalization of Einstein’s theory of gravitation has recently been developed [
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13]. In this theory, the gravitational field is local but satisfies partial integro-differential field equations. Moreover, a significant observational consequence of this theory is that the nonlocal aspect of gravity appears to simulate dark matter. The physical foundations of this classical theory, from nonlocal special relativity theory to nonlocal general relativity, sets it completely apart from purely phenomenological and
ad hoc approaches to the problem of dark matter.
Dark matter is currently required in astrophysics for explaining the gravitational dynamics of galaxies as well as clusters of galaxies [
9], gravitational lensing observations [
10] and structure formation in cosmology [
13]. We emphasize that only some of the implications of nonlocal gravity theory have thus far been confronted with observation [
9,
12]. It is also important to mention here that many other approaches to nonlocal gravitation theory exist that are, however, inspired by developments in quantum field theory. The consideration of such theories is well beyond the scope of this purely classical work.
In this paper, we are concerned with the Newtonian regime of nonlocal gravity, where Poisson’s equation of Newtonian gravity is modified by the addition of a certain average over the gravitational field. This nonlocal term involves a kernel function
q whose functional form can perhaps be derived from a future more complete theory, but, at the present stage of the development of nonlocal gravity, must be determined using observational data. It is necessary that a unique kernel be eventually chosen in this way, but kernel
q at the present time could be either
or
[
6]. Each of these kernels is spherically symmetric in space and contains three length scales
,
, and
such that
. The basic scale of nonlocality is a galactic length
of order 1 kpc, while
is a short-range parameter that controls the behavior of
as
. At the other extreme,
,
decays exponentially as
, indicating the fading of spatial memory with distance. The short-range parameter
is necessary in dealing with the gravitational physics of the Solar System, globular clusters and isolated dwarf galaxies; however, it may be safely neglected in dealing with larger systems such as clusters of galaxies. When
,
and
reduce to a single kernel
,
, and the remaining parameters (
and
) have been determined from a comparison of the theory with the astronomical data regarding a sample of 12 spiral galaxies from the THINGS catalog—see reference [
9] for a detailed treatment. The results can be expressed, for the sake of convenience, as
kpc and
kpc. Moreover, lower limits have been placed on
from the study of the precession of perihelia of planetary orbits in the Solar System [
12,
14,
15].
It is interesting to explore the implications of the virial theorem for nonlocal gravity. In general, the virial theorem of Newtonian physics establishes a simple linear relation between the time averages of the kinetic and potential energies of an isolated material system for which the potential energy is a homogeneous function of spatial coordinates. For an isolated gravitational N-body system, the significance of the virial theorem has to do with the circumstance that the kinetic energy is a sum of terms each proportional to the mass of a body in the system, while the potential energy is a sum of terms each proportional to the product of two masses in the system. Thus, under favorable conditions, the virial theorem can be used to connect the total dynamic mass of an isolated relaxed gravitational system with its average internal motion.
The main purpose of the present paper is to discuss, within the Newtonian regime of nonlocal gravity, the consequences of the extension of the virial theorem to nonlocal gravity. Though such an extension is technically straightforward, it is nevertheless physically quite significant as it allows the possibility of making predictions regarding the effective dark-matter content of cosmologically nearby isolated N-body gravitational systems in virial equilibrium.
2. Modification of the Inverse Square Force Law
It can be shown [
12] that, in the Newtonian regime of nonlocal gravity, the force of gravity on point mass
m due to point mass
is given by:
where
,
and
. The quantity in curly brackets is henceforth denoted by
, where
is the contribution of nonlocality to the force law and depends upon three parameters, namely,
,
and a short-range parameter
that is contained in
; in fact,
when
. We will show in the next section that
starts out from zero at
with vanishing slope and monotonically increases toward an asymptotic value of about 10 as
. Thus, the gravitational force in Equation (
1) is
always attractive; moreover, this force is central, conservative and satisfies Newton’s third law of motion.
Nonlocal gravity is in the early stages of development and, depending on whether we choose kernel
or kernel
,
at the present time can be either
or
respectively, where
,
and
is the
exponential integral function [
16]:
For
,
monotonically decreases from infinity to zero. In fact, near
,
behaves like
and as
,
vanishes exponentially. Furthermore,
where
⋯ is Euler’s constant. It is useful to note that
>(see Equation 5.1.19 in reference [
16]).
It is clear from Equation (
1) that
is dimensionless, while
,
and
have dimensions of length. In fact, we expect that
; moreover, the short-range parameter
and
may be neglected in Equation (
1) when dealing with the rotation curves of spiral galaxies and the internal gravitational physics of clusters of galaxies. In this way,
and
have been
tentatively determined from a detailed comparison of nonlocal gravity with observational data [
9]:
Hence, we find
. It is important to mention here that
is the fundamental length scale of nonlocal gravity at the present epoch; indeed, for
,
and Equation (
1) reduces to Newton’s inverse square force law. In what follows, we usually assume
and
kpc for the sake of convenience. Furthermore, we expect that
is such that
. In reference [
12], preliminary lower limits have been placed on
on the basis of current data regarding planetary orbits in the Solar System. For instance, using the data for the orbit of Saturn, a preliminary lower limit of
cm can be established if we use
, while
cm if we use
.
Therefore,
and
start from zero at
and monotonically increase as
; furthermore, they asymptotically approach
and
, respectively, where
It is a consequence of (
6) that
, so that, in the gravitational force in Equation (
1),
In the Newtonian regime, where we formally let the speed of light
, retardation effects vanish and gravitational memory is purely spatial. The resulting gravitational force in Equation (
1) thus consists of two parts: an enhanced attractive “Newtonian” part and a repulsive fading spatial memory (“Yukawa”) part with an exponential decay length of
kpc. Equation (
1) is such that it reduces to Newton’s inverse square force law for
, as it should [
17,
18,
19,
20,
21], and on galactic scales, it is a generalization of the phenomenological Tohline-Kuhn modified gravity approach to the flat rotation curves of spiral galaxies [
22,
23,
24,
25]. An excellent review of the Tohline-Kuhn work is contained in the paper of Bekenstein [
26].
For
, the exponentially decaying (“fading memory”) part of Equation (
1) can be neglected and
so that
has the interpretation of the
total effective dark mass associated with
. For
, the net effective dark mass associated with point mass
is simply
, where
[
9]. On the other hand, for
, the corresponding result is
, where
depending on whether we use
or
, respectively. The functions in Equation (
13) start from unity at
and decrease monotonically to
and
at
; they are plotted in Figure 1 of reference [
12] for
. If
turns out to be just a few parsecs or smaller, for instance, then
.
A detailed investigation reveals that it is possible to approximate the exterior gravitational force due to a star or a planet by assuming that its mass is concentrated at its center [
12]. In this connection, we note that the radius of a star or a planet is generally much smaller than the length scales
,
and
that appear in the nonlocal contribution to the gravitational force. Therefore, one can employ Equation (
1) in the approximate treatment of the two-body problem in astronomical systems such as binary pulsars and the Solar System, where possible deviations from general relativity may become measurable in the future.
Consider, for instance, the deviation from the Newtonian inverse square force law, namely,
For
, it is possible to show via an expansion in powers of
that [
12]
if
is employed, or
if
is employed. Perhaps dedicated missions, such as ESA’s Gaia mission that was launched in 2013, can measure the imprint of nonlocal gravity in the Solar System [
27,
28]. In this connection, we note that
which, combined with lower limits on
established in reference [
12], is at least three orders of magnitude smaller than the acceleration involved in the Pioneer anomaly (
). It follows from these results that nonlocal gravity is consistent with the gravitational physics of the Solar System.
3. Virial Theorem
Consider an idealized isolated system of
N Newtonian point particles with fixed masses
,
. We assume that the particles occupy a finite region of space and interact with each other only gravitationally such that the center of mass of the isolated system is at rest in a global inertial frame and the isolated system permanently occupies a compact region of space. The equation of motion of the particle with mass
and state
is then
for
, but the case
is excluded in the sum by convention. In fact, a prime over the summation sign indicates that in the sum
. Here,
is a
universal function that is inside the curly brackets in Equation (
1) and the contribution of nonlocality,
, is given by
Consider next the quantities
where
and
It follows from Equation (
18) that
Exchanging
i and
j in the expression on the right-hand side of Equation (
22), we get
Adding Equations (
22) and (
23) results in
Using this result, Equation (
21) takes the form
Let us recall that the net kinetic energy and the Newtonian gravitational potential energy of the system are given by
Hence,
where
and
is given by Equation (
19).
Finally, we are interested in the average of Equation (
27) over time. Let
denote the time average of
f, where
Then, it follows from averaging Equation (
27) over time that
since
, which is the sum of
over all particles in the system, is a bounded function of time and hence the time average of
vanishes. This is clearly based on the assumption that the spatial coordinates and velocities of all particles indeed remain finite for all time. Equation (
30) expresses the
virial theorem in nonlocal Newtonian gravity.
It is important to digress here and re-examine some of the assumptions involved in our derivation of the virial theorem. In general, any consequence of the gravitational interaction involves the whole mass-energy content of the universe due to the universality of the gravitational interaction; therefore, an astronomical system may be considered isolated only to the extent that the tidal influence of the rest of the universe on the internal dynamics of the system can be neglected. Moreover, the parameters of the force law in Equation (
1) refer to the present epoch and hence the virial theorem in Equation (
30) ignores cosmological evolution. Thus, the temporal average over an infinite period of time in Equation (
30) must be reinterpreted here to mean that the relatively isolated system under consideration has evolved under its own gravity such that it is at the present epoch in a steady equilibrium state. That is, the system is currently in virial equilibrium. Finally, we recall that a point particle of mass
m in Equation (
30) could reasonably represent a star of mass
m as well, where the mass of the star is assumed to be concentrated at its center.
The deviation of the virial theorem in Equation (
30) from the Newtonian result is contained in
, where
is given by Equation (
28). More explicitly, we have
It proves useful at this point to study some of the properties of the function
, which is the contribution of nonlocality that is inside the square brackets in Equation (
31). The argument of this function is
for
; therefore,
varies over the interval
, where
is the largest possible distance between any two baryonic point masses in the system. Thus,
, in the context of the virial theorem, is defined for the interval
, where
is the diameter of the smallest sphere that completely encloses the
baryonic system for all time. In general, however,
and
, where
or
, depending on whether we use
or
, respectively. Moreover,
is given by
if we use
or
if we use
. Writing
, where
represents the remainder of the power series, it is straightforward to see that for
and
,
This result, for
and
, implies that the right-hand sides of Equations (
32) and (
33), respectively, are less than
. Therefore, it follows that, in general,
Moreover, for
, (
35) implies:
We conclude that
is a monotonically increasing function of
r that is zero at
with a slope that vanishes at
. For
,
asymptotically approaches a constant
. Here,
is either
or
depending on whether we use
or
, respectively. The functions
and
are defined in Equation (
13).
4. Dark Matter
Most of the matter in the universe is currently thought to be in the form of certain elusive particles that have not been directly detected [
29,
30,
31,
32]. The existence and properties of this
dark matter have thus far been deduced only through its gravity. We are interested here in dark matter only as it pertains to stellar systems such as galaxies and clusters of galaxies [
33,
34,
35,
36,
37,
38,
39]. We mention that dark matter is also essential in the explanation of gravitational lensing observations [
40,
41] and in the solution of the problem of structure formation in cosmology [
13,
42]; however, these topics are beyond the scope of this work.
Actual (mainly baryonic) mass is observationally estimated for astronomical systems using the mass-to-light ratio
. However, it turns out that the dynamic mass of the system is usually larger and this observational fact is normally attributed to the possible existence of nonbaryonic dark matter. Let
M be the baryonic mass and
be the mass of the nonbaryonic dark matter needed to explain the gravitational dynamics of the system. Then,
is the dark matter fraction and
is the dynamic mass of the system.
In observational astrophysics, the virial theorem of Newtonian gravity is interpreted to be a relationship between the dynamic (virial) mass of the entire system and its average internal motion deduced from the rotation curve or velocity dispersion of the bound collection of masses in virial equilibrium. Therefore, regardless of how the net amount of dark matter in galaxies and clusters of galaxies is operationally estimated and the corresponding
is thereby determined, for sufficiently isolated self-gravitating astronomical systems in virial equilibrium, we must have
That is, virial theorem Equation (
38) is employed in astronomy to infer in some way the total dynamic mass of the system. Indeed, Zwicky first noted the need for dark matter in his application of the standard virial theorem of Newtonian gravity to the Coma cluster of galaxies [
33,
34].
5. Effective Dark Matter
A significant physical consequence of nonlocal gravity theory is that it appears to simulate dark matter [
9]. In particular, in the Newtonian regime of nonlocal gravity, the Poisson equation is modified such that the density of ordinary matter
ρ is accompanied by a term
that is obtained from the folding (convolution) of
ρ with the reciprocal kernel of nonlocal gravity. Thus,
has the interpretation of the density of
effective dark matter and
is the density of the
effective dynamic mass.
The virial theorem makes it possible to elucidate in a simple way the manner in which nonlocality can simulate dark matter. It follows from a comparison of Equations (
30) and (
38) that nonlocal gravity can account for this “excess mass” if
where
and
are given in Equations (
26) and (
28), respectively.
It is interesting to apply the virial theorem of nonlocal gravity to sufficiently isolated astronomical N-body systems. The configurations that we briefly consider below consist of clusters of galaxies with diameters kpc, galaxies with and globular star clusters with . The results presented in this section follow from certain general properties of the function and are completely independent of how the baryonic matter is distributed within the astronomical system under consideration.
We emphasize that, after setting the short-range parameter
, the parameters
and
, and hence
, were originally determined from the combined observational data for the rotation curves of a sample of 12 nearby spiral galaxies from the THINGS catalog [
9]. These tentative values are given in Equation (
7). These parameter values were then found to be in reasonable agreement with the internal dynamics of a sample of 10 rich nearby clusters of galaxies from the Chandra X-ray catalog [
9]. In the present paper, we use these parameter values to make predictions about
all nearby isolated N-body gravitational systems that are in virial equilibrium.
5.1. Clusters of Galaxies:
Consider, for example, a cluster of galaxies, where nearly all of the relevant distances are much larger than
kpc. In this case,
and hence
approaches its asymptotic value, namely,
where
or
, defined in Equation (
13), depending on whether we use
or
, respectively. Hence, Equation (
28) can be written as:
It then follows from Equation (
39) that, for galaxy clusters,
in nonlocal gravity. We recall that
ϵ is only weakly sensitive to the magnitude of
. It follows from
that
for galaxy clusters is about 10, in general agreement with observational data [
9]. This theoretical result is essentially equivalent to the work on galaxy clusters contained in reference [
9], except that Equation (
42) takes into account the existence of the short-range parameter
.
Nonlocal gravity thus predicts that the effective dark matter fraction has approximately the same constant value of about 10 for all isolated nearby clusters of galaxies that are in equilibrium.
5.2. Galaxies:
Consider next a sufficiently isolated galaxy of diameter
in virial equilibrium. In this case, we recall that
is a monotonically increasing function of
r, so that for
, Equation (
36) implies
Therefore, it follows from Equation (
28) that, in this case,
The virial theorem for nonlocal gravity in the case of an isolated galaxy is then
which means, when compared with Equation (
38), that
Let us note that
where
is the basic nonlocality length scale. Its exact value is not known; however, from the results of reference [
9], we have
kpc. If we formally let
, then (
46), namely,
, implies that in this case nonlocality and the effective dark matter both disappear, as expected. Therefore, for a sufficiently isolated galaxy in virial equilibrium, the ratio of its baryonic diameter to dark matter fraction
must always be above a fixed length
of about
kpc; that is,
To illustrate (
48), consider, for instance, the Andromeda Galaxy (M31) with a diameter
of about 67 kpc. In this case, we have
[
43,
44], so that for this spiral galaxy
More recently, the distribution of dark matter in M31 has been further studied in reference [
45]. Similarly, for the Triangulum Galaxy (M33), we have
kpc and
[
46], so that
Turning next to an elliptical galaxy, namely, the massive E0 galaxy NGC 1407, we have
kpc and
[
47], so that
Moreover, for the intermediate-luminosity elliptical galaxy NGC 4494, which has a half-light radius of
kpc, the dark matter fraction has been found to be
[
48]. Assuming that the baryonic system has a radius of
, we have
kpc and
; hence,
Let us note that the results presented here are essentially for the present epoch in the expansion of the universe. Observations indicate, however, that the diameters of massive galaxies can increase with decreasing redshift
z. For a discussion of such
massive compact galaxies, see reference [
49].
Finally, it is interesting to consider
at the other extreme, namely, for the case of globular star clusters and isolated dwarf galaxies. The diameter of a globular star cluster is about 40 pc. We can therefore conclude from (
48) with
kpc that for globular star clusters:
Thus, according to the virial theorem of nonlocal gravity, less than about one percent of the mass of a globular star cluster must appear as effective dark matter if the system is sufficiently isolated and is in virial equilibrium. It is not clear to what extent such systems can be considered isolated. It is usually assumed that observational data are consistent with the existence of almost no dark matter in globular star clusters. However, a recent investigation of six galactic globular clusters has led to the conclusion that
[
50]. The resolution of this discrepancy is beyond the scope of the present work.
Isolated dwarf galaxies with diameters
would similarly be expected to contain a relatively small percentage of effective dark matter. There is a significant discrepancy here as well, see reference [
51]; again, the resolution of this difficulty is beyond the scope of this paper. In dwarf systems that are not isolated, the tidal influence of a much larger neighboring galaxy on the dynamics of the dwarf spheroidal galaxy cannot be ignored [
52,
53,
54].